Digital alloy based back barrier for p-channel nitride transistors

ABSTRACT

A III-nitride power handling device and the process of making the III-nitride power handling device are disclosed that use digital alloys as back barrier layer to mitigate the strain due to lattice mismatch between the channel layer and the back barrier layer and to provide increased channel conductivity. An embodiment discloses a GaN transistor using a superlattice binary digital alloy as back barrier comprising alternative layers of AlN and GaN. Other embodiments include using superlattice structures with layers of GaN and AlGaN as well as structures using AlGaN/AlGaN stackups that have different Aluminum concentrations. The disclosed device has substantially increased channel conductivity compared to traditional analog alloy back barrier devices.

TECHNICAL FIELD

The present disclosure is directed in general to the area of III-Nitridetransistors and in particular to the design and fabrication of P-ChannelGaN Transistors.

BACKGROUND OF THE DISCLOSURE

GaN power integrated circuits (IC) have the potential of dramaticallyreducing the size and weight of power electronic systems, therebysubstantially reducing the cost of power electronic devices. P-channelGaN-transistor is a critical component for making GaN power ICs. Suchpower electronic systems are widely needed in electric/hybrid vehicles,more-electric aircrafts, as well as many consumer electronic products.

A variety of GaN transistors are known and they include P-Channeltransistors in conjunction with N-Channel transistors using an analogalloy to improve conductivity. The use of a tertiary alloy such as AlGaN(Aluminum-Gallium-Nitride) is often referred to as analog alloy in theart. To increase power handling capacity of GaN transistors, severaltechniques have been exploited to increase channel conductivity.

It is important to achieve low channel resistance so that the totalpower consumption can be reduced as well as the device speed can beimproved. Increasing the carrier density is one common option that isexploited often. This can be achieved by using high Aluminum (Al)compositions such as AlGaN as back barrier beneath the GaN channel.Higher the aluminum content, higher is the channel conductivity. Howeverthick analog AlGaN with high Al content cannot be grown on GaN bufferdue to the lattice mismatch. This is a fundamental limit of using analogAlGaN alloys as the back-barrier.

The proposed technology overcomes this limitation by proposing a new wayto increase channel conductivity while managing the stress related tolattice mismatch.

SUMMARY OF THE DISCLOSURE

To address one or more of the above-deficiencies of the prior art, oneembodiment described in this disclosure provides for a power handlingdevice comprising, a III-nitride channel layer, a III-nitride cap layeron the channel layer, where the cap layer has higher level of p-typedoping than the channel layer, and a III-nitride digital alloy backbarrier below the channel layer comprising a superlattice orsuperlattice structure. According to an embodiment of the presentdisclosure, a superlattice is a layer consisting of at least threealternating layers of a material A and a material B; each layer ofmaterial A and each layer of material B having a thickness of less than10 nanometer.

Another embodiment described in this disclosure provides for a processof making a power handling device comprising the steps of depositing abuffer layer on a substrate, depositing a digital alloy back barrierlayer on the buffer layer, depositing a III-nitride channel on thedigital alloy layer, depositing a III-nitride p-doped cap layer on thechannel layer, etching a gate recess in the cap layer exposing thechannel layer, depositing a gate dielectric inside the gate recess anddepositing a gate metal inside the gate recess and on the gatedielectric wherein, the digital alloy layer is made up of superlatticestructure.

An embodiment of this disclosure comprises a power handling devicehaving: a III-nitride channel layer, a III-nitride cap layer on thechannel layer, where the cap layer has higher level of p-type dopingthan the channel layer, and a III-nitride digital alloy back barrierbelow the channel layer; the III-nitride digital alloy back barriercomprising a superlattice structure.

According to an embodiment of this disclosure, the power handling devicefurther comprises a buffer layer below the digital alloy back barrierlayer.

According to an embodiment of this disclosure, the III-nitride channellayer is a Gallium Nitride (GaN) channel layer.

According to an embodiment of this disclosure, the cap layer is aMagnesium (Mg) doped GaN.

According to an embodiment of this disclosure, the digital alloy backbarrier is a binary alloy.

According to an embodiment of this disclosure, the superlatticestructure comprises alternating Aluminum Nitride (AlN) and GaN layers.

According to an embodiment of this disclosure, the superlatticestructure comprises ternary alloys.

According to an embodiment of this disclosure, the superlatticestructure comprises alternating GaN and Aluminum-Gallium-Nitride (AlGaN)layers.

According to an embodiment of this disclosure, the superlatticestructure comprises alternating AlGaN and AlGaN layers with differentAluminum percentages.

According to an embodiment of this disclosure, the digital alloy isdesigned to mitigate strain due to lattice mismatch between the channellayer and the digital alloy back barrier layer.

An embodiment of this disclosure also comprises a of making a powerhandling device comprising: depositing a buffer layer on a substrate;depositing a digital alloy back barrier layer on the buffer layer;depositing a III-nitride channel on the digital alloy layer; depositinga III-nitride p-doped cap layer on the channel layer; etching a gaterecess in the cap layer exposing the channel layer; depositing a gatedielectric inside the gate recess; and depositing the gate metal insidethe gate recess and on the gate dielectric, wherein the digital alloylayer is made up of superlattice structure.

According to an embodiment of this disclosure, the III-nitride channelis GaN and the cap layer is a Magnesium (Mg) doped GaN.

According to an embodiment of this disclosure, the digital alloy backbarrier is a binary alloy.

According to an embodiment of this disclosure, the superlatticestructure comprises alternating Aluminum Nitride (AlN) and GaN layers.

According to an embodiment of this disclosure, the superlatticestructure comprises ternary alloys.

According to an embodiment of this disclosure, the superlatticestructure comprises alternating GaN and Aluminum-Gallium-Nitride (AlGaN)layers.

According to an embodiment of this disclosure, the superlatticestructure comprises alternating AlGaN and AlGaN layers with differentAluminum percentages.

According to an embodiment of this disclosure, the digital alloy isdesigned to mitigate the strain due to lattice mismatch between thechannel layer and the digital alloy back barrier layer.

According to an embodiment of this disclosure, the digital alloy layeris between 1 micrometer and 2 micrometer thick.

An embodiment of this disclosure comprises a P-channel III-nitridetransistor having: a III-nitride channel layer, and a III-nitridedigital alloy back barrier below the channel layer; the III-nitridedigital alloy back barrier comprising a superlattice structure.

Certain embodiments may provide various technical advantages dependingon the implementation. For example, a technical advantage of someembodiments may include the use of binary digital alloy such as stackedup layers of Aluminum Nitride and Gallium Nitride. Other embodiments mayuse stack up of ternary digital alloys.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the following figuresand description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates a conventional p-channel GaN structure with analogAlGaN back barrier;

FIG. 2 illustrates a p-channel GaN device containing digital AlN-GaNsuper lattice structure alloys as the back barrier, according to anembodiment of the present disclosure;

FIGS. 3a, 3b and 3c illustrate the first set of steps involved in thefabrication of a p-channel GaN transistor containing digital AlN-GaNsuper lattice alloys as the back barrier, according to an embodiment ofthe present disclosure;

FIGS. 4a, 4b and 4c illustrate the next set of steps involved in thefabrication of a p-channel GaN transistor containing digital AlN-GaNsuper lattice alloys as the back barrier, according to an embodiment ofthe present disclosure;

FIG. 5a illustrates a p-channel GaN transistor structure according to anembodiment of the present disclosure;

FIG. 5b illustrates the channel conductivity characteristics for thep-channel GaN transistor structure illustrated in FIG. 5 a;

FIG. 5c illustrates a p-channel GaN transistor structure according to anembodiment of the present disclosure;

FIG. 5d illustrates the channel conductivity characteristics for thep-channel GaN transistor structure illustrated in FIG. 5 c;

FIG. 6a illustrates a p-channel GaN transistor structure according to anembodiment of the present disclosure;

FIG. 6b illustrates the channel conductivity characteristics for thep-channel GaN transistor structure illustrated in FIG. 6 a;

FIG. 6c illustrates a p-channel GaN transistor structure according to anembodiment of the present disclosure; and

FIG. 6d illustrates the channel conductivity characteristics for thep-channel GaN transistor structure illustrated in FIG. 6 c.

DETAILED DESCRIPTION

It should be understood at the outset that, although example embodimentsare illustrated below, the present technology may be implemented usingany number of techniques, whether currently known or not. The presenttechnology should in no way be limited to the example implementations,drawings, and techniques illustrated below. Additionally, the drawingsare not necessarily drawn to scale.

Recently, GaN transistors have revolutionized high power and high speedswitching electronics and there is a constant push, driven by theapplication demands, for higher and higher power handling capacities.GaN transistor's power handling capacity depends directly on the channelconductivity.

III-Nitride semiconductor materials (Aluminum, Indium, Gallium)Nitrideare excellent wide band gap semiconductors very suitable for modernelectronic and optoelectronic applications. Though this disclosuredescribes the technology using GaN, the proposed technology applies toany of the III-Nitride semiconductor materials.

FIG. 1 illustrates a conventional p-channel GaN transistor structure. Itcontains a ternary alloy (102) Aluminum-Gallium Nitride (AlGaN), whichis normally referred to as analog AlGaN, as the back barrier. This backbarrier 102 is directly under the GaN channel 103 as the back barrierlayer helps to improve the p-type carrier concentration in the GaNchannel, utilizing the polarization effect introduced by the backbarrier AlGaN layer 102. In other words, the higher conductionband_energy in the large bandgap AlGaN barrier layers improves theconfinement of carriers in the low bandgap GaN channel compared to thesame structure without the AlGaN barrier underneath the GaN channel. TheAlGaN layer 102 is built on top of a buffer layer 101 and the bufferlayer helps introducing more holes from the p-type GaN. A layer of GaNcan serve as the buffer layer 101 and can be mounted on a substrate suchas a Sapphire (not shown). A layer 104 of p-doped GaN, (such asMagnesium (Mg) doped GaN) on top of the GaN channel 103 completes theformation of a device that can be used to make a typical GaN transistor.A typical GaN transistor using the structure illustrated in FIG. 1 usesthe polarization effect of the back barrier 102 to increase the channelconductivity. Polarization induced hole density is a known phenomenon inthe art, as the p-type carriers tend to crowd near the polarizedbarrier.

To improve the GaN channel conductivity, there are typically twoapproaches: either increase the carrier density or improve the carriermobility. The carrier mobility depends the interface roughness, filmquality, and other fundamental material properties which are hard tomodify. Thus, increasing the carrier density is a more promisingapproach. The polarization effect in the AlGaN back barrier has a directeffect over the carrier density in the GaN channel. The densityincreases with increasing the polarization in AlGaN, which can berealized by increasing the percentage of Aluminum in the back barrier.

To provide enough polarization effect, AlGaN with reasonable thicknessis required. However, most of the time the AlGaN back barrier is grownon GaN buffer, which has a larger lattice constant compared to AlGaN.This introduces a tensile strain in the AlGaN film. Increasing thepercentage of Aluminum leads to larger tensile strain, which eventuallyresults in cracks in the AlGaN film. Thus, the higher the aluminumconcentration in the AlGaN, smaller the AlGaN thickness would be, toprovide the needed polarization effect. The use of AlGaN as a backbarrier is often referred to as analog back barrier alloy.

To overcome this limitation due to lattice mismatch and increased strainas we increase the aluminum concentration in the back barrier, thisdisclosure proposes a new technology, where the back-barrier layers usedigital alloy instead of the analog alloy to improve the channelconductivity while minimizing the stress related to the latticemismatch. Super lattices such as comprised of stacked up AlN/GaN(Aluminum nitride/Gallium Nitride) alternating layers are referred to asdigital alloys.

FIG. 2 illustrates a GaN device 200, according to an embodiment of thepresent disclosure. It uses alternating AlN/GaN layers forming asuperlattice structure 205, which is referred to as a digital AlGaNalloy, to serve as the back barrier. Superlattice structures help reducethe cracking issues due to lattice mismatches. At the same time,superlattice structures can provide a polarization effect similar towhat the p-type carrier requires to be formed in the GaN channel.

The GaN device 200 comprises of a buffer layer 201, typically about 1.5micrometer thick and a digital superlattice alloy 205 back barriermounted over the buffer layer 201. The super lattice back barrier 205comprises alternating layers of AlN (205 b _(1:n)) and GaN (205 a_(1:n)). This structure is also called as binary alloy. Each of the AlNlayers (205 b _(1:n)) are typically about 2 to 3 nanometer thick, whilethe GaN layers (205 a _(1:n)) can each be typically 2 to 10 nanometerthick. The superlattice (SL) back barrier 205 typically has more thantwo; preferably 30 to 40 pairs of alternating AlN and GaN layers. Analternative SL structure is an alternating AlGaN and GaN stack up 205 orAlGaN/AlGaN stack up 205 where the AlGaN layers have different amountAluminum percentages (by weight or volume).

According to an embodiment of the present disclosure, the superlatticecomprises at least three alternating layers of AlN and GaN; each layerhaving a thickness of less than 10 nanometer.

The GaN channel layer 203 is fabricated over the digital alloy layer205. In a typical device, the GaN channel layer can be 50 nanometer to300 nanometer thick. The Magnesium (Mg) doped GaN cap layer 204 servesto supply the holes to the channel and is fabricated on top of the GaNchannel 203. The p-doped GaN cap layer 204 is typically about 100nanometer to 200 nanometer thick. In this embodiment, the cap layer 204can have much higher Mg doping due to the higher polarization effect ofthe digital alloy layer 205. The overall total device thickness istypically less than 3 micrometer.

The buffer layer 201 can be GaN or AlGaN. The AlGaN/GaNhigh-electron-mobility transistor requires a thermally conducting,semi-insulating substrate to achieve the best possible performance. Thesemi-insulating SiC substrate is currently the best choice for thisdevice technology; however, fringing fields which penetrate the GaNbuffer layer at pinch-off introduce significant substrate conduction atmodest drain bias if channel electrons are not well confined to thenitride structure. The addition of an insulating buffer on thesemi-insulating SiC substrate suppresses this parasitic conduction,which results in dramatic improvements in the AlGaN/GaN transistorperformance. A pronounced reduction in both the gate-lag and thegate-leakage current are observed for structures with the buffer layer.

The digital SL alloy back barrier layer 205 provides the necessarypolarization effect along the interface with the GaN channel layer 203.Holes crowd along this interface to provide for the necessary channelconductivity. The digital alloy helps providing a thick back barrierwith a strong polarization maintained, which helps introduce more holesinto the GaN channel layer 203. A transistor made using GaN device 200was evaluated for performance and found to have substantially higherswitching speeds, lower switching loss and a substantially reducedparasitic value.

The process of making a transistor using the GaN device 200 can comprisefabricating the layers as structured in FIG. 3b , followed by theprocess steps illustrated in FIGS. 3c and 4a through 4c. The digitalalloy 305 can be fabricated as illustrated in FIG. 3a . The alloy 305can either be fabricated first or alternatively fabricated as part offabricating the stack as illustrated in FIG. 3 b.

The digital alloy 305 can be formed by stacking up alternative layers ofGaN and AlN. A GaN layer is formed using one of many known techniques inthe art. One such technique is molecular beam epitaxy (MBE).Alternatively, a GaN layer can also be formed by a sputtering process ata high substrate temperature—for example greater than 700-degreeCelsius. Since Gallium is a liquid at around 30-degree Celsius, initialnitridation of Gallium liquid metal surface needs to be done in thissputtering process using pure Ga as target surface. A horizontal watercooled stainless steel trough can be used for the growth of GaNepilayer. A reactive Direct Current (DC) magnetron sputter epitaxy canbe used. In fabricating the digital alloy 305, a GaN layer is firstformed to the required thickness of between 2 nanometer to 10 nanometer,followed by an AlN (Aluminum Nitride) layer formed on top of this GaNlayer by several known techniques, such as sputtering, electroplating orMBE. Typically, a 2 nanometer to 3 nanometer thick AlN layer is formed.The process is repeated for subsequent pair of GaN and AlN layers. It isnot unusual to form 30 to 40 pairs of GaN/AlN layers by repeating thisprocess, yielding a digital alloy that is less than 3 micrometer thick.

The process of making the device 200 comprises forming a buffer layer301 on a substrate 306 of choice, such as Silicon Carbide (SiC) orSaphire, as illustrated in FIG. 3b . The buffer layer 301 can be anysuitable material, such as GaN or AlGaN. The buffer layer is typicallyabout 1 to 2 micrometer thick and preferably 1.5 micrometer thick. Thedigital alloy 305 is formed on top of this buffer layer as describedearlier. Alternatively, the digital alloy can be fabricated separatelyand deposited on top of the buffer layer by any known techniques such asMBE or a sputtering process. Typically, the digital alloy will bebetween 1 to 2 micrometer thick and preferably about 1.5 micrometerthick. The GaN channel is fabricated on top of the digital alloy usingtechniques described earlier. The GaN channel is typically between 50nanometer to 300 nanometer thick, and preferably 100 nanometer thick.The p-doped GaN layer is formed on top of the GaN channel by any knownprocess, such as MBE or sputtering. In a preferred embodiment, Magnesium(Mg) is used as the p-dopant.

The next step in the process is to etch a gate recess 307 that exposesthe GaN channel 303 as illustrated in FIG. 3c . This etching processtypically comprises masking the area around and exposing the etchingarea and using known etching techniques (such as using chemical or gasor laser/molecular beams) to etch out the gate area and exposing a partof the GaN channel 303. Next, a suitable gate dielectric 408 such asSilicon dioxide is deposited as illustrated in FIG. 4a by any knowntechniques, such as depositing Silicon by sputtering followed byoxidation or any other suitable technique depending on the chosen gatedielectric.

FIG. 4b illustrates the next step of forming the ohmic contacts 409 aand 409 b on the p-channel. This step comprises of depositing thecontact electrode of the device, such as gold, copper or silver and istypically done using electroplating or sputtering process. The finalstep is to form the gate metal in the cavity of the dielectric asillustrated in FIG. 4c . The gate metal can be gold, silver or copper orany other suitable gate material. Depending on the chosen gate material,process such as electroplating or sputtering can be used. The surfacescan be polished, cleaned and separated from the substrate to form thedevice 200.

In the process steps described above, any of the steps illustrated canbe changed, modified or eliminated to better suit the materials chosenor as known to one in the art. Additional steps may be added as neededto perfect the device fabrication per specification.

Several test fabrications and characterization of the proposedtechnology were carried out to characterize and quantify the benefits ofthe proposed technology. Since the density of hole along the channel isone of the prime factor deciding the power handling capacity of thedevice, the tests measure current flow as a function of applied voltagefor various device structures. FIGS. 5a-5d and FIGS. 6a-6d illustratethe results of this study. The structures are illustrated on the left(FIGS. 5a, 5c, 6a and 6c ) while the performance characteristics of therespective structures are illustrated on the right (FIGS. 5b, 5d, 6b and6d ).

FIG. 5a illustrates a GaN transistor structure having an epi structurewithout the back barrier. It comprises of the Buffer layer 501 andhaving a GaN channel 503 on top. The p-doped GaN layer 504 using Mg asthe dopant is deposited on top of the GaN channel 503. The ohmiccontacts 509 a and 509 b is used to apply a voltage to the device andcurrent through the structure is measured. On the right is the measuredperformance characteristics of this structure as shown in FIG. 5b . TheX-axis 512 is the voltage applied in Volts between the ohmic contacts509 a and 509 b. The Y-axis 511 is the measured current through thedevice in μamps. It can be noted that as the voltage is varied between−10 v to +10 v, the current through the device as shown by the curve 521linearly increases between −150 μamps to +150 μamps. This baselineperformance can be compared with the performance of the subsequentimprovements in the device structure.

FIG. 5c illustrates a revised device structure where an analog alloy 502is added to the device. This analog alloy 502 creates the polarizationeffect along the GaN channel boundary causing hole crowding andincreased current flow. FIG. 5d illustrates the performance of thisstructure in FIG. 5c . For the same range of applied voltage (−10 v to+10 v), the current increases to −410 μamps to +410 μamps as shown bythe performance curve 522. This is almost a threefold increase in thedevice conductivity, primarily caused by the introduction of the analogalloy 502.

Next, we compare the performance of having a AlN spacer 612, which isprimarily introduced between the GaN channel 503 and the AlGaN layer 502as illustrated in FIG. 6a , to mitigate the stress due to the latticemismatch. This spacer 612 marginally increases the current flow as shownby the curve 623 to about −580 μamps to +580 μamps as the appliedvoltage between the ohmic contacts is varied in the same range of −10 vto +10 v. The structure in FIG. 6a has a marginally better performance623 in FIG. 6b compared with the performance curves 521and 522. Thespacer 612 also helps mitigate the stress due to lattice mismatchbetween the GaN layer 503 and the analog alloy 502.

Finally, we illustrate the substantial performance improvement of theproposed technology. FIG. 6c illustrates the digital alloy layer 605replacing the analog alloy 502, located between the Buffer layer 501 andthe GaN channel 503. The corresponding performance curve 624 in FIG. 6dshows the current through the device in the range of −1.0 milliamps to+1.0 milliamps as the applied voltage is varied between the same voltagerange of −10 v to +10 v, as shown by FIG. 6d . This is almost a 3×performance in device conductivity compared to the analog alloyperformance 522 in FIG. 5d . One can titrate the thickness of thedigital alloy 605 and/or the material and/or individual alloy materialthicknesses of the alloy compositions to increase the conductivityfurther, while mitigating the stress due to lattice mismatch between theGaN channel 503 and the digital alloy layer 605.

In particular configurations, it may be desirable to have the AlN/GaNalloy to create the polarization effect to enhance hole crowding. Inother configurations, one can use AlGaN/GaN alloy to create thepolarization effect. The device stack up can be varied to suit theintended applications. Thickness of each layers as well as the digitalalloy thickness can be varied to meet particular performance needs.Though this disclosure describes the technology using GaN, the proposedtechnology applies to any of the III-Nitride semiconductor materials(Al, In, Ga)N or the combinations thereof

Modifications, additions, or omissions may be made to the systems,apparatuses, and methods described herein without departing from thescope of the inventive concepts. The components of the systems andapparatuses may be integrated or separated. Moreover, the operations ofthe systems and apparatuses may be performed by more, fewer, or othercomponents. The methods may include more, fewer, or other steps.Additionally, steps may be performed in any suitable order. As used inthis document, “each” refers to each member of a set or each member of asubset of a set.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists onthe date of filing hereof unless the words “means for” or “step for” areexplicitly used in the particular claim.

1-10. (canceled)
 11. A process of making a power handling devicecomprising: depositing a buffer layer on a substrate; depositing adigital alloy back barrier layer on the buffer layer; depositing aIII-nitride channel on the digital alloy layer; depositing a III-nitridep-doped cap layer on the channel layer; etching a gate recess in the caplayer exposing the channel layer; depositing a gate dielectric insidethe gate recess; and depositing the gate metal inside the gate recessand on the gate dielectric wherein, the digital alloy layer is made upof superlattice structure.
 12. The process of making the power handlingdevice of claim 11, wherein the III-nitride channel is GaN and whereinthe cap layer is a Magnesium (Mg) doped GaN.
 13. The process of makingthe power handling device of claim 11, wherein the digital alloy backbarrier is a binary alloy.
 14. The process of making the power handlingdevice of claim 11, wherein the superlattice structure comprisesalternating Aluminum Nitride (AlN) and GaN layers.
 15. The process ofmaking the power handling device of claim 11, wherein the superlatticestructure comprises ternary alloys.
 16. The process of making the powerhandling device of claim 11, wherein the superlattice structurecomprises alternating GaN and Aluminum-Gallium-Nitride (AlGaN) layers.17. The process of making the power handling device of claim 11, whereinthe superlattice structure comprises alternating AlGaN and AlGaN layerswith different Aluminum percentages.
 18. The process of making the powerhandling device of claim 11, wherein the digital alloy is designed tomitigate the strain due to lattice mismatch between the channel layerand the digital alloy back barrier layer.
 19. The process of making thepower handling device of claim 11, wherein the digital alloy layer isbetween 1 micrometer and 2 micrometer thick.
 20. (canceled)